This invention relates to reflective materials. Specifically, this invention relates to reflective materials known as light directing materials. More specifically, this invention relates to light directing materials based on substrates containing nanopores.
Light directing materials are used in a number of applications. Light directing materials can use reflection or refraction to direct the incident light in the desired direction. The most common type returns the incident light in the angle of incident (retroreflective). Least common and most complicated are surface relief holograms which direct reflections to form an image which appears three-dimensional. Retroreflective surfaces are commonly used in applications where low light levels can create safety hazards. These applications include use in road signs and safety clothing. In many states, the use of retroreflective material in road signage is mandated by law. Retroreflective surfaces are typically superior to other reflective surfaces because retroreflective surfaces reflect a portion of the incident light striking their surfaces in rays parallel to the incident rays. Flat surfaces, however, display this property only for light that is normal to the surface. In all other circumstances, light is reflected from a flat surface at an angle of reflection that is equal to the angle of incidence from the surface normal, but opposite in direction to the angle of incidence. Therefore, retroreflective surfaces are generally more highly visible when illuminated from the point of the observer.
Several approaches are currently used to create retroreflective surfaces. One such approach is the use of spherical micro-bubbles or beads made of glass or other materials to form a retroreflective layer. U.S. Pat. Nos. 5,128,804, 5,207,852, 5,695,853, 5,853,846, and 5,882,771 disclose retroreflective materials of this type. This type of surface 10, is schematically displayed in FIG. 1. Typically, glass microspheres or microbubbles 11 are applied over a release sheet or a clear carrier sheet 12 in a clear base resin. This coating is kept very thin to minimize internal scattering and absorption of light by the microspheres. This thin coating creates a textured surface, the hard glass spheres protruding from the coating as the coating dries or chemically cures and shrinks. A layer of aluminum 13 or other metal is evaporated on the textured surface to create spherical micro-reflectors on the back of the sheet. A portion of the light entering the smooth side is reflected back at the angle of incidence from the spherical micro-mirrors on the back of the sheet. An adhesive 14 is applied onto the metal.
Light striking this type of surface may be reflected in one of two ways. First, due to the differences in reflective index between the glass and the polymer resin, a portion of the incident light is reflected off the surface of the sphere, as at 15. The close packed spheres reflect this light between the spheres and a very small portion of the incident light is returned to the light source. Second, light enters the microspheres and reflects off the spherical micro-mirrors at the back of the sheet, a portion of this light, as at 16. A portion of the reflected light is then refocused to return the rays at the angle of incident.
Another approach used for retroreflective surfaces utilizes a honeycomb type structure 18 as shown in FIG. 2. The backing 19 of the honeycomb structure is made from a plastic such as PVC. The depressions in the honeycomb structure are then filled with spherical colloidal metal particles or metallized glass spheres 20 in a monolayer. A clear top sheet 21 is bonded to the honeycomb sheet and adhesive 22 and backing material 19 are then applied. Similar to the microsphere material 10, the incident light rays 23 are reflected off the metal surfaces of the close packed spheres and a portion returned at the angle of incidence.
Another retroreflective material type utilizes corner cube design and total internal reflection to create a retroreflective effect. This design 25 is represented in FIG. 3. In this type of material, the back surface of the material has a repeating corner cube structure. This structure appears as the shape of the corner of a cube 26, which has been sliced off of a cube. Incident light enters through the flat front surface and strikes one facet 27 of the corner cube. Visible light 28 is almost totally reflected off the facet surface due to the refractive index of the cube material and air 29. The light is reflected to one or more of the other facets in such a way that a portion of the light is reflected back toward the light source. One example of the use of this design is demonstrated in U.S. Pat. No. 5,660,768.
It should be noted that only a portion of the incident light striking any of the above described retroreflective surfaces is transmitted back toward the source. Other portions are absorbed by the materials comprising the reflective surface or are reflected in other directions. Depending on the materials used, a portion of the incident light may also be transmitted through the optical materials of the retroreflective surface or metal layer.
Examples of these types of retroreflective surfaces are commercially available. These products, however, exhibit certain undesirable characteristics. Among these are the use of a rigid rather than a flexible support backing, increasing the difficulty of transporting the product and using it in certain circumstances. Such material, for example, cannot be easily stored in rolls or applied to curved surfaces. Another drawback regarding currently available materials relates to the propensity of some plastics to absorb water. Over time, water migrates through the material, causing hazing and cracking of the material during temperature cycling. Therefore, it would be advantageous to be able to use a water resistant base material for a retroreflective surface. It would also be advantageous to use a material that can be coated with a water resistant coating such as an aliphatic polyurethane or a polyolefin. Finally, it would be advantageous to develop a retroreflective article that maximized the amount of light it reflected.